When a little girl is kidnapped, and her kidnapper turns up dead, Batman must turn to the only other person who could possibly know her whereabouts: the Joker.
Batman is speeding towards Arkham Asylum to follow up a lead on a case. As he arrives, he visits the Joker in his cell and interrogates him about the perpetrator - Sherman Stroud - who used to be confined next to his cell.
Batman is convinced that Stroud has taken a page out of the Joker's playbook in his latest crime so he asks for his help to solve the case. The Joker reveals that he has influenced Stroud during his incarceration and gave him an idea - to kidnap a little girl. After a round of mind games, he reveals that the girl is trapped in the trunk of a car on a ferry.
The Dark Knight races to the harbor and is met by a bunch of goons. He confronts them then attempts to rescue the little girl as the ferry sinks beneath the waves from an explosion.
After the rescue, Batman returns to Joker's cell. Baffled by his cooperation, he asks the Joker why. The Joker correctly assumes that Batman didn't expect the girl to be alive. It turns out it's a lesson on hope.